1. Field of the Invention
The invention relates to electronic weighing devices. More particularly, the invention relates to an electronic weighing device which employs surface acoustic waves to measure weight.
2. State of the Art
Precision electronic weighing devices are widely known in the art and there are many different technologies utilized in these electronic weighing devices. Laboratory scales or "balances" typically have a capacity of about 1,200 grams and a resolution of about 0.1 gram, although scales with the same resolution and a range of 12,000 grams are available. The accuracy of these scales is achieved through the use of a technology known as magnetic force restoration. Generally, magnetic force restoration involves the use of an electromagnet to oppose the weight on the scale platform. The greater the weight on the platform, the greater the electrical current needed to maintain the weight. While these scales are very accurate (up to one part in 120,000), they are expensive and very sensitive to ambient temperature. In addition, their range is relatively limited.
Most all other electronic weighing devices use load cell technology. In load cell scales, the applied weight compresses a column which has strain gauges bonded to its surface. The strain gauge is a fine wire which undergoes a change in electrical resistance when it is either stretched or compressed. A measurement of this change in resistance yields a measure of the applied weight. Load cell scales are used in non-critical weighing operations and usually have a resolution of about one part in 3,000. The maximum resolution available in a load cell scale is about one part in 10,000 which is insufficient for many critical weighing operations. However, load cell scales can have a capacity of several thousand pounds.
While there have been many improvements in electronic weighing apparatus, there remains a current need for electronic weighing apparatus which have enhanced accuracy, expanded range, and low cost.
Co-owned application Ser. No. 08/489,365, previously incorporated by reference herein, discloses an electronic weighing apparatus having a base which supports a cantilevered elastic member upon which a load platform is mounted. The free end of the elastic member is provided with a first piezoelectric transducer and a second piezoelectric transducer is supported by the base. Each transducer includes a substantially rectangular piezoelectric substrate and a pair of electrodes imprinted on the substrate at one end thereof, with one pair of electrodes acting as a transmitter and the other pair of electrodes acting as a receiver. The transducers are arranged with their substrates substantially parallel to each other with a small gap between them and with their respective electrodes in relatively opposite positions. The receiver electrodes of the second transducer are coupled to the input of an amplifier and the output of the amplifier is coupled to the transmitter electrodes of the first transducer. The transducers form a "delay line" and the resulting circuit of the delay line and the amplifier is a positive feedback loop, i.e. a natural oscillator. More particularly, the output of the amplifier causes the first transducer to emit a surface acoustic wave ("SAW") which propagates along the surface of the first transducer substrate away from its electrodes. The propagating waves in the first transducer induce an oscillating electric field in the substrate which in turn induces similar SAW waves on the surface of the second transducer substrate which propagate in the same direction along the surface of the second transducer substrate toward the electrodes of the second transducer. The induced waves in the second transducer cause it to produce an alternating voltage which is supplied by the electrodes of the second transducer to the amplifier input. The circuit acts as a natural oscillator, with the output of the amplifier having a particular frequency which depends on the physical characteristics of the transducers and their distance from each other, as well as the distance between the respective electrodes of the transducers.
When a load is applied to the load platform, the free end of the cantilevered elastic member moves and causes the first transducer to move relative to the second transducer. The movement of the first transducer relative to the second transducer causes a change in the frequency at the output of the amplifier. The movement of the elastic member is proportional to the weight of the applied load and the frequency and/or change in frequency at the output of the amplifier can be calibrated to the displacement of the elastic member. The frequency response of the delay line is represented by a series of lobes. Each mode of oscillation is defined as a frequency where the sum of the phases in the oscillator is an integer multiple of 2.pi.. Thus, as the frequency of the oscillator changes, the modes of oscillation move through the frequency response curve and are separated from each other by a phase shift of 2.pi.. The mode at which the oscillator will oscillate is the one having the least loss. The transducers are arranged such that their displacement over the weight range of the weighing apparatus causes the oscillator to oscillate in more than one mode. Therefore, the change in frequency of the oscillator as plotted against displacement of the transducers is a periodic function. There are several different ways of determining the cycle of the periodic function so that the exact displacement of the elastic member may be determined. In addition, in order to minimize the possibility that the oscillator will oscillate in two modes at the same time, the frequency response of the delay line is arranged so that no more than two modes coexist in the main lobe of the frequency response curve. This is achieved by the topology of the electrodes as well as the distance between the transmitting electrode and the receiving electrode. The gain of the amplifier is also chosen to be at least the absolute value of the greatest loss expected to be encountered at an oscillating frequency within the main lobe but not great enough to allow oscillation in two modes simultaneously.
According to a disclosed preferred embodiment, the surface acoustic wave has a wavelength of approximately 200 microns at 20 MHz. The gap between the substrates of the first and second transducers is as small as possible and preferably is less than 0.1 wavelength, i.e. 10-20 microns. The amplifier preferably has a gain of at least approximately 17 dB in order to guarantee natural oscillation, and preferably not more than approximately 30 dB so that the oscillator oscillates in only one mode at a time. The preferred manner of determining the cycle of the periodic output of the amplifier is to provide a second pair of transducers adjacent to the first pair and coupled to each other in the same type of delay line feedback loop. The second pair of transducers utilize a SAW with a different wavelength than the first pair of transducers, e.g. approximately 220 microns at 18 MHz. The output of the second amplifier is, therefore, a periodic function with a different frequency than the periodic function which is the output of the first amplifier. By combining the outputs of both amplifiers, a unique value is provided for each position of the elastic member.
Typically, the elastic member is chosen so that it will bend up to 150 microns under maximum load. Given the wavelength of the SAW, this results in about two to three modes of oscillation in the output of the first amplifier.
The provided apparatus can theoretically achieve an accuracy on the order of one part in one hundred thousand, e.g. one gram per hundred kilograms. In practice, however, a resolution on the order of one part in fifty thousand is readily achieved. It has been observed by the inventors herein that several factors have varying influence on the accuracy of the SAW system. These factors include reflected waves, temperature changes, and the frequency of the oscillator. Generally, reflected waves result in non-linearity of measurements, and temperature has an effect of about 70 ppm per degree C.